Large-scale wastewater disposal has led to a fast-paced reawakening of faults in the Oklahoma/Kansas region. High-resolution earthquake relocations show that the inventory of ancient basement faults in the study region differs from results of seismic surveys and geologic mapping focused on the sedimentary cover. We analyze the evolution of seismic activity in the Guthrie-Langston sequence in central Oklahoma in greater detail. Here, seismic activity has reactivated a network of at least 12 subvertical faults in an area less than 10 km across. Recorded activity began in late 2013, peaked about six months later, and includes two M 4 earthquakes. These earthquakes characteristically occur at about 4 km depth below the top of the basement and do not reach the sedimentary cover. The sequence shows a radial growth pattern despite being no closer than 10 km to significant wastewater disposal activity. Hydrologic modeling suggests that activity initiated with a time lag of several years relative to early injection activity. Once initiated, earthquake interactions contribute to the propagation of seismicity along the reactivated faults. As a result, the spatiotemporal evolution of the seismicity mimics a diffusive pattern that is typically thought to be associated with injection activity. Analysis of the fault slip potential shows that most faults are critically stressed in the contemporary stress field. Activity on some faults, for which we find low slip probability, suggests a significant contribution of geomechanical heterogeneities to the reawakening of these ancient basement faults.

Documentation and analysis of groundwater responses to induced earthquakes are important to better understand their influence on shallow groundwater systems and hydrogeological properties and processes. Here we show that induced seismicity in Oklahoma can cause changes of groundwater level over distances >150 km from the epicenter. We test existing models for the cause of the observed responses and find that the model most consistent with observations is enhanced crustal permeability produced by seismic waves, changing aquifer recharge. Simulation suggests that the sources of this recharge are close to the responding wells and have lateral dimensions of ∼100 m. Continuous monitoring of pressure and temperature in wells, installing clustered wells to monitor multiple water levels near injection sites, and isotopic and chemical analysis of groundwater near injection sites are required to better understand and quantify the recharging sources.